Pyruvic Acid

Pyruvic acid and its derivative phosphoenol pyruvate have already appeared in Figure 1.1, and pyruvic acid was required in discussion of the action of thiamine diphosphate (Figures 2.12 to 2.14). They are important intermediates and will appear again. It is worth looking briefly at the origins of these compounds now.

When glucose is broken down in metabolism, the first steps are to convert glucose to glucose 6-phosphate, which is isomerized to fructose 6-phosphate and then converted to fructose 1,6-bisphosphate (Figure 2.20). This is cleaved by an aldol reaction running in reverse, and catalyzed by the enzyme aldolase. The products are dihydroxyacetone 1-phosphate and glyceraldehyde 3-phosphate, which are really equivalent compounds because they are interconverted through a common enol form. Glyceraldehyde 3-phosphate is phosphorylated again and oxidized to glyceric acid 1,3-bisphosphate, linked to the conversion of one molecule of NAD+ to NADH. Glyceric acid bisphosphate loses one phosphate to ADP, forming ATP while the 3-phosphate is isomerized to 2-phosphate. Loss of water from this compound gives phosphoenol pyruvate and transfer of the phosphate to ADP gives another molecule of ATP, and conversion of enolpyruvate to the keto-form gives pyruvic acid. The summary in Figure 2.20 does not give all the stages, nor considers the mechanisms or the energetics of this important process. For that the reader is referred to a standard textbook of biochemistry.

Under anaerobic conditions, the NADH produced during the oxidation of glyceraldehyde to glyceric acid is re-oxidized in the reduction of pyruvic acid to lactic acid (in bacteria), or in the reduction of acetalde-hyde to ethanol (in yeast); and, under aerobic conditions, is oxidized (via

glucose 6-phosphate

(p)= phosphate

fructose 1,6-bisphosphate

oh pyruvic acid h2C-0-(p) ho-ch2

Dihydroxyacetone phosphate hç=0 hç-oh h2c-o-(p)

glyceraldehyde phosphate

nad+

oh 0

glyceric acid bisphosphate

-h20

oh o phosphoenol pyruvate

Figure 2.20 A summary of the steps by which pyruvic acid and phosphoenol pyruvate are obtained from glucose during metabolism. The key step is a retro-aldol reaction on fructose 1,6-bisphosphate the citric acid cycle) to produce more energy in higher animals. Glycerol is produced by reduction of glyceraldehyde 3-phosphate.

Some insects are cold-hardy, that is, they have evolved ways of surviving very low temperatures at which their blood might freeze and the ice-crystals pierce their cell walls. One way is for insects to produce large quantities of glycerol which lowers the freezing point of their blood. Glycerol concentration can reach 2 M or more, representing 20% of the fresh body weight of the insect in winter months. It has been shown that the cold-hardy gall moth Epiblema scudderiana produces a special aldolase that converts fructose 1,6-bisphosphate into glycerol. The enzyme is more active at 5 °C than at 20 °C.